Poly(ethylene terephthalate) ("PET"), nylons, such as Nylon 6 and Nylon 66, and Rayon are the predominant synthetic polymers used in making tire yarn and tire cord. Each polymer has its own advantages and disadvantages. The most widely used of these, PET, has high tensile strength and tensile modulus, a high glass transition temperature, and good stability. Nylon has excellent strength, toughness and fatigue resistance, but has the serious disadvantage of "flat spotting" in tires because of its low glass transition temperature and a tendency to creep. Rayon retains a higher percentage of its tensile properties at elevated temperatures (e.g., 150.degree. C.). Considerable research has been carried out in an effort to improve the properties of these materials.
One approach has been to try to synthesize a polyester which has a higher tensile strength and tensile modulus than PET and retains these properties at elevated temperatures. Such a material would retain the inherent advantages of polyesters in general, such as chemical stability. Alternative polyesters that have been made and evaluated include poly(ethylene naphthalate) ("PEN"), the condensation polymer of ethylene glycol and 2,6-naphthalenedicarboxylic acid, and the polymer of 4,4'-bibenzoic acid and ethylene glycol.
Copolymers in which 4,4'-bibenzoic acid and/or 2,6-naphthalene dicarboxylic acid are included as comonomers in PET have been reported in European Patent Application No. 202,631. A copolymer of 4,4'-bibenzoic acid, 2,6-naphthalenedicarboxylic acid, and ethylene glycol was reported in Japanese Published Patent Application 50-135333 to be particularly useful for making tire yarn when the mole ratio of 4,4'-bibenzoic acid to 2,6-naphthalenedicarboxylic acid is less than about 1:4. This reference states that when 4,4'-bibenzoic acid makes up more than about 20 mole % of the diacids in the composition, the composition is of no value as a tire yarn because it has a low softening temperature and a low Young's modulus (tensile modulus). This conclusion is supported by examples which indicate that the softening temperature has decreased from 275.degree. C. for PEN to 238.degree. C. in the copolymer in which monomer units derived from 4,4'-bibenzoic acid make up 25% of the monomer units derived from the two diacids (i.e. the ratio of monomer units derived from 4,4'-bibenzoic acid to those derived from 2,6-naphthalenedicarboxylic acid is 1:3).
Japanese Patent Application Kokai No. 52-107319 (laid open Sep. 8, 1977) teaches polyester fibers formed from a polyester derived from acid components comprised mainly of diphenyldicarboxylic acid (A) and naphthalene dicarboxylic acid (B) wherein (A) comprises 20-80 mole % thereof and (B) comprises 80-20 mole % thereof, and glycol components comprised mainly of ethylene glycol. This polyester had an intrinsic viscosity of less than 0.9 dl/g.
U.S. Pat. No. 3,008,934 (Wielicki, et al.) describes a polyester of bibenzoic acid, a dihydric alcohol, and certain aromatic acids. Example 7 teaches fiber made from a polyethylene bibenzoate/2,6-naphthalene-dicarboxylate (50/50 mole %). This polyester des not have a high molecular weight; consequently, the fiber has less than optimal tensile properties.